Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Feedback-ID: ieff94742:Fastmail
User-Agent: Cyrus-JMAP/3.7.0-alpha0-757-gc3ad9c75d3-fm-20220722.001-gc3ad9c75
Mime-Version: 1.0
Message-Id: <7cec93c5-3db4-409b-8c1e-bc1f10dd68fc@www.fastmail.com>
In-Reply-To: <20220614120231.48165-11-kirill.shutemov@linux.intel.com>
References: <20220614120231.48165-1-kirill.shutemov@linux.intel.com>
 <20220614120231.48165-11-kirill.shutemov@linux.intel.com>
Date:   Tue, 26 Jul 2022 13:17:13 -0700
From:   "Andy Lutomirski" <luto@kernel.org>
To:     "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        "Borislav Petkov" <bp@alien8.de>,
        "Sean Christopherson" <seanjc@google.com>,
        "Andrew Morton" <akpm@linux-foundation.org>,
        "Joerg Roedel" <jroedel@suse.de>,
        "Ard Biesheuvel" <ardb@kernel.org>
Cc:     "Andi Kleen" <ak@linux.intel.com>,
        "Sathyanarayanan Kuppuswamy" 
        <sathyanarayanan.kuppuswamy@linux.intel.com>,
        "David Rientjes" <rientjes@google.com>,
        "Vlastimil Babka" <vbabka@suse.cz>,
        "Tom Lendacky" <thomas.lendacky@amd.com>,
        "Thomas Gleixner" <tglx@linutronix.de>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        "Paolo Bonzini" <pbonzini@redhat.com>,
        "Ingo Molnar" <mingo@redhat.com>,
        "Varad Gautam" <varad.gautam@suse.com>,
        "Dario Faggioli" <dfaggioli@suse.com>,
        "Dave Hansen" <dave.hansen@intel.com>,
        "Mike Rapoport" <rppt@kernel.org>,
        "David Hildenbrand" <david@redhat.com>,
        "Marcelo Henrique Cerri" <marcelo.cerri@canonical.com>,
        tim.gardner@canonical.com, khalid.elmously@canonical.com,
        philip.cox@canonical.com,
        "the arch/x86 maintainers" <x86@kernel.org>, linux-mm@kvack.org,
        linux-coco@lists.linux.dev, linux-efi@vger.kernel.org,
        "Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCHv7 10/14] x86/mm: Avoid load_unaligned_zeropad() stepping into
 unaccepted memory
Content-Type: text/plain;charset=utf-8
Content-Transfer-Encoding: quoted-printable
Precedence: bulk


On Tue, Jun 14, 2022, at 5:02 AM, Kirill A. Shutemov wrote:
> load_unaligned_zeropad() can lead to unwanted loads across page bounda=
ries.
> The unwanted loads are typically harmless. But, they might be made to
> totally unrelated or even unmapped memory. load_unaligned_zeropad()
> relies on exception fixup (#PF, #GP and now #VE) to recover from these
> unwanted loads.
>
> But, this approach does not work for unaccepted memory. For TDX, a load
> from unaccepted memory will not lead to a recoverable exception within
> the guest. The guest will exit to the VMM where the only recourse is to
> terminate the guest.

Why is unaccepted memory marked present in the direct map in the first p=
lace?

Having kernel code assume that every valid address is followed by severa=
l bytes of memory that may be read without side effects other than #PF a=
lso seems like a mistake, but I probably won=E2=80=99t win that fight. B=
ut sticking guard pages in front of definitely-not-logically present pag=
es seems silly to me.  Let=E2=80=99s just not map it.

(What if MMIO memory is mapped next to regular memory?  Doing random una=
ligned reads that cross into MMIO seems unwise.)